Big business on Tuesday upped its pressure on Members and the White House to reach a deal to raise the nation&rsquo;s debt limit to avoid what it views as a potential fiscal calamity.

&ldquo;Now is the time for our political leaders to put aside partisan differences and act in the nation&rsquo;s best interests,&rdquo; more than 450 CEOs wrote in a letter organized by the U.S. Chamber of Commerce and other business organizations. The letter went to all Members of Congress and to the White House.

&ldquo;It is time to pull together rather than pull apart,&rdquo; they wrote.

A source who worked on the letter said privately that the missive is the tip of a much larger, behind-the-scenes lobbying effort to educate Members of Congress, particularly freshman Republicans, on the perils of not raising the debt limit. The chamber&rsquo;s Bruce Josten and other top business lobbyists have been privately meeting with Members for months, the source added.

The source said the letter Tuesday has been in the works for a couple of weeks.

Chamber President Tom Donohue said in a statement announcing the letter that an &ldquo;unprecedented default on the nation&rsquo;s bills would have dire consequences for our economy, our markets, and Main Street Americans.&rdquo;

His group, along with the National Association of Manufacturers, the Business Roundtable and the Financial Services Forum, helped craft the letter and organized the CEOs and business executives who signed it. Those top executives include Mark Bertolini of Aetna Inc., Stephen Schwarzman of the Blackstone Group, Caterpillar Inc.&rsquo;s Doug Oberhelman, Vikram Pandit of Citigroup Inc., General Mills Inc.&rsquo;s Kendall Powell, Robert Koch II of Koch Enterprises, Stanley Black &amp; Decker Inc.&rsquo;s John Lundgren, UPS Inc.&rsquo;s Scott Davis, Matt Rose of BNSF Railway Co., Michael Duke of Wal-Mart Stores Inc. and other business leaders.

Rose and Davis were on a list of four business leaders who the White House said were having lunch with President Barack Obama on Tuesday to discuss jobs and the economy.

&ldquo;The business community in large numbers is saying to our leaders in Washington, &lsquo;Do your job,&rsquo;&rdquo; Business Roundtable President John Engler said in the statement. &ldquo;Failure to raise the debt ceiling would strike an immediate and serious blow to any economic recovery, and failure to make significant progress on long-term debt reduction will continue the uncertainty which is hampering our investment climate.&rdquo;

Tuesday&rsquo;s letter is the business groups&rsquo; second missive on the issue to Members and the White House, but the most recent one is more pointed and has about 400 more signatures.

&ldquo;Failure to raise the debt ceiling and the ensuing default and inability of our country to pay its bills as they come due would have harsh implications for the dollar, the international and domestic financial system, economic growth and job creation,&rdquo; Financial Services Forum President and CEO Rob Nichols said in the statement. &ldquo;It is critically important that our leaders arrive at a deal to avoid both the negative consequences of a default and address our federal debt and large annual budget deficits in a responsible way.&rdquo;

The source involved in the letter said the business groups&rsquo; next step is unclear and that grass-roots or advertising campaigns are not immediately in the works. &ldquo;You have asked the question. I don&rsquo;t know the answer,&rdquo; this source said. &ldquo;We just have to keep communicating that default is not an option because the economic consequences would be so disastrous.&rdquo;

Another source said the business groups would not be able to push for the deal until the White House and Congressional leaders actually come to an agreement.